The present invention relates to a color printer printing a color image by the electronic photographing method, and more particularly, to a color printer provided with optical beam scanning units and photosensitive drums in numbers corresponding to the number of kinds of color used for color synthesis.
Recent progress of laser printers toward color printers has been remarkable, and is resulting in the emergence of low-cost compact color printers. In order to achieve a color laser printer, it is necessary to use a multiple transcription method based on a plurality of colors. Among other multiple transcription methods, there is known the tandem method, which comprises providing an optical beam scanning unit and a photosensitive drum for each color to be printed. For the YMCK system (yellow, magenta, cyanogen, and black), for example, four sets of an optical beam scanning unit and a photosensitive drum are provided. In the case of such a tandem type color laser printer, scanning lines must be created for each of the YMCK colors to be printed, and a strict color shift accuracy must be complied with: a positional accuracy of scanning line of under 100 .mu.m is required for a resolution of 600 dpi, for example.
In a conventional tandem type color laser printer, however, the necessary positional accuracy of scanning lines for each color to be printed is maintained through adjustment upon assembling each optical beam scanning unit. As a result, an adjustment operation in the manufacturing stage requires much time and labor, thus leading to a higher cost.
Because of the use of optical beam scanning units in a number corresponding to the number of colors to be printed, a trouble, if any, in an optical beam scanning unit for a particular color, naturally requiring replacement of the troubled optical unit, requires a positioning adjustment again as in the manufacturing stage, and the difficult replacement of each optical unit has thus been a problem in the conventional tandem type color laser printer.